1. Field of the Invention
The present invention relates to a method and a device for calibrating a fuel metering system of an internal combustion engine, in particular of a motor vehicle.
2. Description of the Related Art
In today's fuel injection systems of the type included here, for example, in common-rail diesel injection systems, to improve a mixture formation, partial injections having relatively small fuel quantities are carried out at the same time before a corresponding main injection. The mentioned main injection is generally calculated based on a torque request of a corresponding driver. The injection quantities of the mentioned partial injections are to be as small as possible to avoid emission disadvantages. On the other hand, the injection quantities must be sufficiently large so that, in consideration of all tolerance sources, the minimum quantity necessary for the corresponding combustion process is always discharged. A mixture formation improved in this way allows reduced exhaust gas emissions and decreased combustion noises.
The small fuel quantities in the case of the mentioned partial injections require precise metering of the particular injection quantities. If a partial injection is entirely omitted, for example, because a provided injection component, in the case of common-rail injection systems an injector, has not yet injected as a result of routine tolerances in the case of an underlying control signal, this has substantial effects on the operation of the internal combustion engine, which is expressed, for example, by increased noise development during the combustion.
In the case of the mentioned common-rail diesel injection systems, pressure generation and injection are decoupled from one another with the aid of a high-pressure accumulator, a so-called “rail,” the injection pressure being generated independently of the engine speed and the injection quantity and being available for the injection in the high-pressure accumulator. The particular injection timing and the particular injection quantity are calculated in an electronic engine control unit and are metered by the corresponding injectors of each cylinder of the internal combustion engine via remote-controlled valves. It is to be ensured that the mentioned partial injections are always implemented with the highest possible precision.
Manufacturing tolerances occurring during the manufacturing of injectors of a corresponding fuel metering system cause differences in the operating characteristics of the individual injectors, which often only occur over the lifetime of the particular injectors or of the fuel metering system or are even amplified during the lifetime. In addition, the injectors of a fuel metering system typically have different quantity characteristics maps, i.e., different dependencies between injection quantities, rail pressure, and activation duration. This has the result that the various injectors also fill the corresponding combustion chamber with different quantities of fuel in the event of a very precise activation.
A substantial tolerance source for the quantity precision of the pilot injection is therefore the so-called “drift” of the particular injector, i.e., the operating characteristics, which continuously change over the lifetime of the injector.
Metering of the mentioned minimal quantities is carried out based on a so-called zero-fuel quantity calibration. This is described, for example, in the publication published German patent application document DE 199 45 618 A1. In so-called “overrun mode” of the particular internal combustion engine, a single injector is activated and the activation duration is gradually increased until, in the event of a minimum activation duration, a change of a quantity replacement signal results, for example, a torque increase measurable on the internal combustion engine, on the basis of which it is recognizable that an injection has now occurred. The activation duration then provided corresponds to an operating state, in the case of which the injection just begins for the affected internal combustion engine, i.e., the cylinder of the internal combustion engine. This procedure is carried out accordingly with respect to all injectors or cylinders of the internal combustion engine. The nominal activation duration, i.e., the activation duration which would result without application of the function, is subtracted from the activation duration values thus obtained and the difference is stored in a nonvolatile way. During a subsequent activation of the injectors in the fired mode, the stored differences are added as activation duration correction values to the value which would result without use of the present function.
Furthermore, a method and a device for calibrating a fuel metering system of an internal combustion engine are known from published German patent application document DE 10 2008 002 482 A1, in which at least one injector is activated using a first test injection having a first test activation duration and a resulting first quantity signal is detected. The at least one injector is then activated using at least one second test injection having a second activation duration, which deviates from the first activation duration, and at least one second quantity signal thus resulting is detected. On the basis of the first minimum activation duration and the at least one second minimum activation duration and the first quantity signal and the at least one second quantity signal, a regression calculation is then carried out. The learning method during the zero-fuel quantity calibration may then be improved with the aid of the method provided therein, in that the time required for the learning of a calibration value is decreased.
In addition, a method and a device for calibrating the injection quantity of a partial injection in an injection system of an internal combustion engine are described in published German patent application document DE 10 2008 043 165 A1, in which a correction value for a partial injection in an individual cylinder of the internal combustion engine is ascertained by stimulating an injection pattern and by changing a speed variation of the internal combustion engine caused by the injection pattern. For this purpose, a quantity redistribution is carried out by changing over the injection pattern at a preferably stationary load point, typically in the idle state. For this purpose, the rail pressure is intentionally preset to particular calibration pressures. The calibration of the individual injectors at the individual rail pressure stages is carried out sequentially.
The calibration methods described in the first two publications may only be carried out in the above-mentioned overrun mode of the internal combustion engine. Such an operating state is not available in the case of some internal combustion engine types, for example, hybrid engines.
The method disclosed in the last-mentioned publication may also be carried out without overrun mode, for example, in driving mode of an internal combustion engine of the motor vehicle. However, the adjustment of the rail pressure, in particular the changeover of the injection pattern, is acoustically perceptible and is therefore not acceptable in the field of passenger automobiles as a result of the reduced driving comfort.